Introduction

The pharynx is part of both the respiratory and digestive system. Both systems have entrances to the pharynx but they are separated from each other by the soft palate.
During exercise or during respiratory distress, the mouth can be used as an additional opening of the respiratory system and then the oropharynx also becomes an air-way.

The pharynx can be split into different regions - the oropharynx, laryngeal pharynx (sometimes referred to as the oesophageal pharynx) and the nasopharynx.

Nasopharynx Anatomy - Copyright RVC

Structure and Function

The opening of the larynx is on the floor of the pharynx. Caudal and dorsal to the laryngeal opening is the opening into the oesophagus. In the dorsal region of the nasopharynx, there are paired openings into the auditory (eustachian) tubes. The lining of the middle ear cavity and auditory tube is continuous with that of the nasopharynx. The pharynx is located between the base of the skull and the first two cervical vertebrae dorsally and the larynx ventrally. The mandible, pterygoid muscles and hyoid apparatus are located laterally. Laterally, two pairs of palatopharyngeal arches are present from the soft palate to the oesophagus.

Pharynx Labelled - Copyright C.Clarkson and T.F.Fletcher University of Minnesota

The walls of the pharynx contain striated muscle. During swallowing, the soft palate is raised which divides the pharynx into dorsal and ventral sections. The dorsal compartment is the nasopharynx and the rostral compartment is the oropharynx. The laryngeal pharynx is separated from the oropharynx by the epiglottis. Tonsils are present on the lateral walls of the oropharynx and are covered by flaps of mucosa.

Musculature

Pharyngeal muscles arise from arch 4 except for the stylopharyngeous muscle that comes from arch 3.

Muscles that constrict

Muscles that constrict the pharynx run dorsally to its roof. Rostral constrictor muscles are the hyopharyngeous, pterygopharyngeous and the palatopharyngeous muscles. They originate from the pterygoid region of the skull and insert on the aponeurosis of the soft palate and shorten the pharynx. The middle constrictor muscle is the thyopharyngeous muscle. It originates from the hyoid bone. Caudally is the cricopharyngeous muscle, which originates from the thyroid cartilage of the larynx.

Muscles that dilate

Muscles that dilate the pharynx enclose it laterally and dorsally. The dilator muscle is the stylopharyngeous muscle. It originates from the hyoid apparatus and widens the rostral pharynx.

Muscles that shorten

The pterygopharyngeal muscle and palatopharyngeal muscle shorten the pharynx. They enclose the pharynx laterally and dorsally.

Muscles that close the Pharyngeal Arch

The palatopharyngeous muscle also closes the pharyngeal arch.

Innervation

The nervous supply comes from the pharyngeal branch of the vagus nerve (CN X) and from the cranial root of the accessory nerve (CN XI). The stylopharyngeous muscle is innervated by the accessory nerve (CN XI). The glossopharyngeal nerve (CN IX) supplies taste to the pharynx.

Histology

Fibroelastic aponeurosis supports the mucosa. The nasopharynx has pseudostratified columnar epithelium. The oropharynx and the laryngeal pharynx have stratified squamous epithelium. The oropharynx and the laryngeal pharynx also have salivary glands present.

Species Differences

Palatine Tonsil - Copyright C.Clarkson and T.F.Fletcher University of Minnesota

Canine

A single duct connects the nasopharynx to the nasal cavity. The tonsils are a compact mass which point away from the lumen of the pharynx.

Equine

The auditory tube opens into the guttural pouch and the tonsils are diffuse and raised slightly. Horses are unable to breathe through the mouth as the free apex of the rostral epiglottis lies dorsal to the soft palate in a normal horse.

Ruminants

Tonsils are a compact mass which point towards the lumen of the pharynx.